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To evaluate the environmental impact of uranium (U) contamination, it is important to investigate the effects of U at ecologically relevant conditions. Since U speciation, and hence its toxicity, strongly depends on environmental pH, the present study aimed to investigate dose-dependent effects of U at pH 7.5. Arabidopsis thaliana plants (Mouse-ear Cress) were exposed for three days to different U concentrations at pH 7.5. In the roots, the increased capacities of ascorbate peroxidase and glutathione reductase indicate an important role for the ascorbate-glutathione cycle during U-induced stress. However, a significant decrease in the ascorbate redox state was observed after exposure to 75 and 100 µM U, indicating that those roots are severely stressed. In accordance with the roots, the ascorbate-glutathione cycle plays an important role in the antioxidative defence systems in A. thaliana leaves exposed to U at pH 7.5 as the ascorbate and glutathione biosynthesis were upregulated. In addition, small inductions of enzymes of the antioxidative defence system were observed at lower U concentrations to counteract the U-induced stress. However, at higher U concentrations it seems that the antioxidative defence system of the leaves collapses as reductions in enzyme activities and gene expression levels were observed. [ABSTRACT FROM AUTHOR]

Uranium (U) causes oxidative stress in Arabidopsis thaliana plants grown at pH 5.5. However, U speciation and its toxicity strongly depend on environmental parameters, for example pH. It is unknown how different U species determine U uptake and translocation within plants and how they might affect the oxidative defense mechanisms of these plants. The present study analyzed U uptake and oxidative stress-related responses in A. thaliana (Columbia ecotype) under contrasted U chemical speciation conditions. The 18-d-old seedlings were exposed for 3 d to 25 µM U in a nutrient solution of which the pH was adjusted to 4.5, 5.5, 6.5, or 7.5. Results indicate that there is a different rate of U uptake and translocation at the different pHs, with high uptake and low translocation at low pH and lower uptake but higher translocation at high pH. After U exposure, an increased glutathione reductase activity and total glutathione concentration were observed in U-exposed roots, pointing toward an important role for glutathione in the root defense system against U either by chelation or by antioxidative defense mechanisms. In leaves, antioxidative defense mechanisms were activated on U exposure, indicated by increased superoxide dismutase and catalase activity. As it seems that U toxicity is influenced by pH, it is important to consider site-specific characteristics when making U risk assessments. Environ Toxicol Chem 2013;32:2125-2133. © 2013 SETAC [ABSTRACT FROM AUTHOR]

This study aimed to investigate effects on growth and development and alterations in the nutrient profiles for Arabidopsis thaliana seedlings following uranium exposure. Seventeen-day-old Arabidopsis thaliana seedlings, grown in hydroponics, were exposed to 0, 0.1, 1, 10 and 100 μM uranium for one, three and seven days. Fresh weight of leaves and roots decreased after exposure to 100 μM uranium for one and three days and after exposure to 1, 10 and 100 μM uranium for seven days. Anthocyanous-colored leaves and stunted yellow roots were observed after exposure to 100 μM uranium. The uranium content of the roots highly increased but the root-to-shoot transfer of uranium was limited. Uranium exposure resulted in disturbed macro- and micronutrient profiles, especially following exposure to 100 μM. This study indicates that elevated uranium concentrations can cause important morphological and physiological effects and disturb the nutrient profiles in Arabidopsis thaliana seedlings. [ABSTRACT FROM PUBLISHER]

Abstract: When aiming to evaluate the environmental impact of uranium contamination, it is important to unravel the mechanisms by which plants respond to uranium stress. As oxidative stress seems an important modulator under other heavy metal stress, this study aimed to investigate oxidative stress related responses in Arabidopsis thaliana exposed to uranium concentrations ranging from 0.1 to 100 μM for 1, 3 and 7 days. Besides analyzing relevant reactive oxygen species-producing and -scavenging enzymes at protein and transcriptional level, the importance of the ascorbate–glutathione cycle under uranium stress was investigated. These results are reported separately for roots and leaves in two papers: Part I dealing with responses in the roots and Part II unraveling responses in the leaves and presenting general conclusions. Results of Part I indicate that oxidative stress related responses in the roots were only triggered following exposure to the highest uranium concentration of 100 μM. A fast oxidative burst was suggested based on the observed enhancement of lipoxygenase (LOX1) and respiratory burst oxydase homolog (RBOHD) transcript levels already after 1 day. The first line of defense was attributed to superoxide dismutase (SOD), also triggered from the first day. The enhanced SOD-capacity observed at protein level corresponded with an enhanced expression of iron SOD (FSD1) located in the plastids. For the detoxification of H2O2, an early increase in catalase (CAT1) transcript levels was observed while peroxidase capacities were enhanced at the later stage of 3 days. Although the ascorbate peroxidase capacity and gene expression (APX1) increased, the ascorbate/dehydroascorbate redox balance was completely disrupted and shifted toward the oxidized form. This disrupted balance could not be inverted by the glutathione part of the cycle although the glutathione redox balance could be maintained. [Copyright &y& Elsevier]

Abstract: Uranium never occurs as a single pollutant in the environment, but always in combination with other stressors such as ionizing radiation. As effects induced by multiple contaminants can differ markedly from the effects induced by the individual stressors, this multiple pollution context should not be neglected. In this study, effects on growth, nutrient uptake and oxidative stress induced by the single stressors uranium and gamma radiation are compared with the effects induced by the combination of both stressors. By doing this, we aim to better understand the effects induced by the combined stressors but also to get more insight in stressor-specific response mechanisms. Eighteen-day-old Arabidopsis thaliana seedlings were exposed for 3 days to 10 μM uranium and 3.5 Gy gamma radiation. Gamma radiation interfered with uranium uptake, resulting in decreased uranium concentrations in the roots, but with higher transport to the leaves. This resulted in a better root growth but increased leaf lipid peroxidation. For the other endpoints studied, effects under combined exposure were mostly determined by uranium presence and only limited influenced by gamma presence. Furthermore, an important role is suggested for CAT1/2/3 gene expression under uranium and mixed stressor conditions in the leaves. [Copyright &y& Elsevier]

Abstract: In this study, toxicity effects in plants of uranium in a binary pollution condition were investigated by studying biological responses and unraveling oxidative stress related mechanisms in Arabidopsis thaliana seedlings, grown on hydroponics and exposed for 3 days to 10μM uranium in combination with 5μM cadmium. While uranium mostly accumulated in the roots with very low root-to-shoot transport, cadmium was taken up less by the roots but showed higher translocation to the shoots. Under mixed exposure, cadmium influenced uranium uptake highly but not the other way round resulting in a doubled uranium concentration in the roots. Under our mixed exposure conditions, it is clear that micronutrient concentrations in the roots are strongly influenced by addition of cadmium as a second stressor, while leaf macronutrient concentrations are mostly influenced by uranium. Oxidative stress related responses are highly affected by cadmium while uranium influence is more limited. Hereby, an important role was attributed to the ascorbate redox balance together with glutathione as both metabolites, but more explicitly for ascorbate, increased their reduced form, indicating an important defense and regulatory function. While for roots, based on an increase in FSD1 gene expression, oxidative stress was suggested to be superoxide induced, in leaves on the other hand, hydrogen peroxide related genes were mostly altered. [ABSTRACT FROM AUTHOR]

Abstract: Arabidopsis thaliana was exposed to low-dose chronic gamma irradiation during a full life cycle (seed to seed) and several biological responses were investigated. Applied dose rates were 2336, 367 and 81 μGy h−1. Following 24 days (inflorescence emergence), 34 days (∼50% of flowers open) and 54 days (silice ripening) exposure, plants were harvested and monitored for biometric parameters, capacities of enzymes involved in the antioxidative defence mechanisms (SOD, APOD, GLUR, GPOD, SPOD, CAT, ME), glutathione and ascorbate pool, lipid peroxidation products, altered gene expression of selected genes encoding for antioxidative enzymes or reactive oxygen species production, and DNA integrity. Root fresh weight was significantly reduced after gamma exposure compared to the control at all stages monitored but no significant differences in root weight for the different dose rates applied was observed. Leaf and stem fresh weight were significantly reduced at the highest irradiation level after 54 days exposure only. Also total plant fresh was significantly lower at silice riping and this for the highest and medium dose rate applied. The dose rate estimated to result in a 10% reduction in growth (EDR-10) ranged between 60 and 80 μGy h−1. Germination of seeds from the gamma irradiated plants was not hampered. For several of the antioxidative defence enzymes studied, the enzyme capacity was generally stimulated towards flowering but generally no significant effect of dose rate on enzyme capacity was observed. Gene analysis revealed a significant transient and dose dependent change in expression of RBOHC indicating active reactive oxygen production induced by gamma irradiation. No effect of irradiation was observed on concentration or reduction state of the non-enzymatic antioxidants, ascorbate and glutathione. The level of lipid peroxidation products remained constant throughout the observation period and was not affected by dose rate. The comet assay did not reveal any effect of gamma dose rate on DNA integrity. [Copyright &y& Elsevier]

Abstract: The production of reactive oxygen species (ROS) and the induction of the antioxidative defense mechanism are very important in heavy metal toxicity. In this study, biological effects induced after uranium contamination were investigated for Arabidopsis thaliana. Three-week-old seedlings were exposed for 4days to 100μM U in an adjusted Hoagland solution. Uranium exposure caused a decreased growth of leaves (38%) and roots (70%) and a modified nutrient profile was observed. Investigation of lipid peroxidation products indicated a significant increase of membrane damage. Important ROS-producing and -scavenging enzymes were studied at transcriptional and protein level to investigate the importance of the ROS-signature in uranium toxicity. Elevated gene expression was observed for NADPH-oxidase, a ROS-producing enzyme. Changes in gene expression for different ROS-scavenging enzymes as Cu/ZnSOD, FeSOD and APX were also observed. Analysis of enzyme capacities showed little effects after uranium contamination. Higher ascorbate levels in uranium exposed leaves suggested an increase of antioxidative defense via the ascorbate-glutathione pathway after uranium exposure. Theoretical calculations indicated rapid formation of uranium-phosphate precipitates if normal phosphate concentrations are used. Precipitation tests recommend the use of 25μM P in combination with 100μM U to inhibit uranium precipitation. Because this combination was used for uranium toxicity investigation, the influence of this low phosphate concentration on plant growth and oxidative stress had to be evaluated. Minor differences between low phosphate (25μM P) and high phosphate (100μM P) treatments were observed justifying the use of the low phosphate concentration in combination with uranium. [Copyright &y& Elsevier]

When terrestrial environments get contaminated with long-lived gamma emitting radionuclides, plants that grow in these contaminated areas are exposed to gamma radiation during consecutive generations. Therefore it is important to evaluate the gamma induced stress response in plants in and between generations. The objective of this research is to reveal differences at the level of the antioxidative stress response between generations with a different radiation history. An experiment was conducted in which 7-days old Arabidopsis thaliana plants were exposed for 14 days to four different gamma dose rates: 22 mGy/h, 38 mGy/h, 86 mGy/h and 457 mGy/h. Two different plant groups were used: plants that were not exposed to gamma radiation before (P0) and plants that received the aforementioned gamma treatment during their previous generation (S1). Growth, the concentration of the antioxidants ascorbate and glutathione, a number of antioxidative enzyme activities and their gene transcript levels were analysed. A dose-rate dependent induction was seen for catalase (CAT) and guaiacol peroxidase (GPX) in the roots and for syringaldazine peroxidase (SPX) in the shoots. Differences between the two generations were observed for CAT and GPX in the roots, where a significantly higher activity of these reactive oxygen species (ROS) detoxifying enzymes was observed in the S1 generation. For SPX in the shoots, a dose dependent upregulation was observed in the P0 generation. However, high SPX activities were present for all doses in the S1 generation. These differences in enzyme activity between generations for SPX and GPX and the involvement of these enzymes in cell wall biosynthesis, suggest an important role for cell wall strengthening in the response to gamma irradiation. [ABSTRACT FROM AUTHOR]

Anthropogenic activities have led to a widespread uranium (U) contamination in many countries. The toxic effects of U at the cellular level have mainly been investigated at a pH around 5.5, the optimal pH for hydroponically grown plants. However, since the speciation of U, and hence its toxicity, is strongly dependent on environmental factors such as the pH, it is important to investigate the effects of U at different environmentally relevant pH levels. Although U is poorly translocated from the roots to the shoots, resulting in a low U concentration in the leaves, it has been demonstrated that toxic effects in the leaves were already visible after 1 day exposure at pH 5.5, although only when exposed to relatively high U concentrations (100 μM). Therefore, the present study aimed to analyse the effects of different U concentrations (ranging from 0 to 100 μM) at pH 4.5 in leaves of Arabidopsis thaliana plants. Results indicate that U induces early senescence in A. thaliana leaves as was suggested by a decreased expression of CAT2 accompanied by an induction of CAT3 expression, a decreased CAT capacity and an increased lipid peroxidation. In addition, miRNA398b/c is involved in the regulation of the SOD response in the leaves. As such, an increased MIR398b/c expression was observed leading to a decreased transcript level of CSD1 / 2 . Finally, the biosynthesis of ascorbate was induced after U exposure. This can point towards an important role for this metabolite in the scavenging of reactive oxygen species under U stress. [ABSTRACT FROM AUTHOR]

Uranium (U) is a nonessential and toxic heavy metal and radioactive element. Nitric oxide (NO) and hydrogen peroxide (H 2 O 2 ) are intimately involved in the growth, development and metabolic function of plant cells. In addition to possible toxic activities, a role for these signalling molecules in the regulation of plant responses to toxic metals has been proposed. Here, we investigated the synthesis of NO and H 2 O 2 in Arabidopsis thaliana seedlings exposed to 25 μM U. We demonstrated that U induces NO and H 2 O 2 production in the roots and leaves. Addition of the NO releasing compound, sodium nitroprusside (SNP) as well as the NO synthase inhibitor (L-NAME) to the plants, resulted in a decrease of the U-induced NO production and both alleviated U toxicity as indicated by plant growth (leaf area and biomass). Treatment with L-NAME further resulted in the disappearance of the U-induced H 2 O 2 in the shoots of the plants. It appears that generation of NO and H 2 O 2 is intimately linked in the toxicity response of U-exposed A. thaliana plants. [ABSTRACT FROM AUTHOR]

Human activity has led to an increasing amount of radionuclides in the environment and subsequently to an increased risk of exposure of the biosphere to ionising radiation. Due to their high linear energy transfer, α-emitters form a threat to biota when absorbed or integrated in living tissue. Among these, 241 Am is of major concern due to high affinity for organic matter and high specific activity. This study examines the dose-dependent biological effects of α-radiation delivered by 241 Am at the morphological, physiological and molecular level in 14-day old seedlings of Arabidopsis thaliana after hydroponic exposure for 4 or 7 days. Our results show that 241 Am has high transfer to the roots but low translocation to the shoots. In the roots, we observed a transcriptional response of reactive oxygen species scavenging and DNA repair pathways. At the physiological and morphological level this resulted in a response which evolved from redox balance control and stable biomass at low dose rates to growth reduction, reduced transfer and redox balance decline at higher dose rates. This situation was also reflected in the shoots where, despite the absence of a transcriptional response, the control of photosynthesis performance and redox balance declined with increasing dose rate. The data further suggest that the effects in both organs were initiated in the roots, where the highest dose rates occurred, ultimately affecting photosynthesis performance and carbon assimilation. Though further detailed study of nutrient balance and 241 Am localisation is necessary, it is clear that radionuclide uptake and distribution is a major parameter in the global exposure effects on plant performance and health. [ABSTRACT FROM AUTHOR]

The biosphere is constantly exposed to ionising radiation, due to cosmic radiation and the presence of natural and anthropogenic radionuclides in the environment. The biological effects of the resulting radiation exposure are currently poorly understood in plants, due to a large influence of parameters such as species, cultivar, experimental setup and plant age on the response. In this study, 7-, 10- and 14-day - old Arabidopsis thaliana seedlings were exposed to 96 h and 168 h of 100 mGy h −1 chronic γ-radiation in a hydroponic setup. The response of growth, photosynthesis and the transcription of genes involved in DNA repair, cell cycle and signalling were measured. The results show that there is a difference in growth response between plants of different ages and decreasing radiosensitivity with increasing seedling age. This is linked to differences in regulation of DNA repair and cell cycle control at the transcriptional level. [ABSTRACT FROM AUTHOR]

To study the impact of environmental uranium (U) contamination, effects should be analysed at different environmentally relevant pH levels as the speciation of U, and hence its toxicity, is strongly dependent on the pH. As photosynthesis is a major energy producing process in plants intimately connected to plant growth and known to be susceptible to metal stress, the effects of different U concentrations on photosynthesis in 18-day-old Arabidopsis thaliana (Columbia ecotype) are investigated at two contrasting pH levels, pH 4.5 and pH 7.5. At pH 4.5, U is highly taken up by the roots but is poorly translocated to the shoots, while at pH 7.5, less U is taken up but the translocation is higher. The lower U concentrations in the shoots at pH 4.5 are accompanied by a more reduced leaf growth as compared to pH 7.5. In addition, U does not influence the photosynthetic machinery at pH 7.5, while an optimization of the photosynthesis takes place after U exposure at pH 4.5. As such, more of the absorbed quanta are effectively used for photosynthesis accompanied by a decreased non-photochemical quenching and an increased electron transport rate. Since the enhanced photosynthesis at pH 4.5 is accompanied by a decreased growth, we suggest that the energy produced during photosynthesis is used for defence reactions against U-induced oxidative stress rather than for growth. As such, a high discrepancy was observed between the two pH levels, with an optimized photosynthetic apparatus at pH 4.5 and almost no effects at pH 7.5. [ABSTRACT FROM AUTHOR]

Abstract: There is a need for a better understanding of biological effects of radiation exposure in non-human biota. Correct description of these effects requires a more detailed model of dosimetry than that available in current risk assessment tools, particularly for plants. In this paper, we propose a simple model for dose calculations in roots and shoots of Arabidopsis thaliana seedlings exposed to radionuclides in a hydroponic exposure setup. This model is used to compare absorbed doses for three radionuclides, 241Am (α-radiation), 90Sr (β-radiation) and 133Ba (γ radiation). Using established dosimetric calculation methods, dose conversion coefficient values were determined for each organ separately based on uptake data from the different plant organs. These calculations were then compared to the DCC values obtained with the ERICA tool under equivalent geometry assumptions. When comparing with our new method, the ERICA tool appears to overestimate internal doses and underestimate external doses in the roots for all three radionuclides, though each to a different extent. These observations might help to refine dose–response relationships. The DCC values for 90Sr in roots are shown to deviate the most. A dose–effect curve for 90Sr β-radiation has been established on biomass and photosynthesis endpoints, but no significant dose-dependent effects are observed. This indicates the need for use of endpoints at the molecular and physiological scale. [Copyright &y& Elsevier]

Abstract: To obtain a better understanding on how non-human biota are affected by exposure to environmental radioactivity, it is essential to link observed effects to a correct estimate of absorbed ionising radiation dose. Current wildlife dose rate and risk assessment tools are not set up to assess changes in dose rate during organism development. This paper presents a dosimetry model for assessing dose rate and absorbed dose during seedling development of the model plant Arabidopsis thaliana. We included growth and radionuclide absorption dynamics into the dose calculations. This model was subsequently used to compare the dose and dose rate calculations for three radionuclides, 241Am (α-radiation), 90Sr (β-radiation) and 133Ba (γ-radiation), in a standard exposure scenario. We show that growth influences dose and dose rate and that this influence depends on the radionuclide and the organ involved. The use of dynamic dosimetry models greatly improves the dose calculations for effect studies. [Copyright &y& Elsevier]

Abstract: As the environment is inevitably exposed to ionizing radiation from natural and anthropogenic sources, it is important to evaluate gamma radiation induced stress responses in plants. The objective of this research is therefore to investigate radiation effects in Arabidopsis thaliana on individual and subcellular level by exposing 2-weeks-old seedlings for 7 days to total doses of 3.9Gy, 6.7Gy, 14.8Gy and 58.8Gy and evaluating growth, photosynthesis, chlorophyll a, chlorophyll b and carotenoid concentrations and antioxidative enzyme capacities. While the capacity of photosystem II (PSII measured as F v/F m) remained intact, plants started optimizing their photosynthetic process at the lower radiation doses by increasing the PSII efficiency (φPSII) and the maximal electron transport rate (ETRmax) and by decreasing the non-photochemical quenching (NPQ). At the highest radiation dose, photosynthetic parameters resembled those of control conditions. On subcellular level, roots showed increased superoxide dismutase (SOD) and ascorbate peroxidase (APX) capacities under gamma irradiation but catalase (CAT), syringaldazine peroxidase (SPX) and guaiacol peroxidase (GPX) activities, on the other hand, decreased. In the leaves no alterations were observed in SOD, CAT and SPX capacities, but GPX was highly affected. Based on these results it seems that roots are more sensitive for oxidative stress under gamma radiation exposure than leaves. [Copyright &y& Elsevier]

Highlights: [•] Uranium is highly taken up by roots but concentrations in leaves remain low. [•] Increase efficiency of photosystem II under uranium stress. [•] Decrease non-photochemical quenching under uranium stress. [•] Increase in photosynthetic rate in the light-limited region of rapid light curve. [Copyright &y& Elsevier]

Abstract: The cellular redox balance seems an important modulator under heavy metal stress. While for other heavy metals these processes are well studied, oxidative stress related responses are also known to be triggered under uranium stress but information remains limited. This study aimed to further unravel the mechanisms by which plants respond to uranium stress. Seventeen-day-old Arabidopsis thaliana seedlings, grown on a modified Hoagland solution under controlled conditions, were exposed to 0, 0.1, 1, 10 and 100 μM uranium for 1, 3 and 7 days. While in Part I of this study oxidative stress related responses in the roots were discussed, this second Part II discusses oxidative stress related responses in the leaves and general conclusions drawn from the results of the roots and the leaves will be presented. As several responses were already visible following 1 day exposure, when uranium concentrations in the leaves were negligible, a root-to-shoot signaling system was suggested in which plastids could be important sensing sites. While lipid peroxidation, based on the amount of thiobarbituric acid reactive compounds, was observed after exposure to 100 μM uranium, affecting membrane structure and function, a transient concentration dependent response pattern was visible for lipoxygenase initiated lipid peroxidation. This transient character of uranium stress responses in leaves was emphasized by results of lipoxygenase (LOX2) and antioxidative enzyme transcript levels, enzyme capacities and glutathione concentrations both in time as with concentration. The ascorbate redox balance seemed an important modulator of uranium stress responses in the leaves as in addition to the previous transient responses, the total ascorbate concentration and ascorbate/dehydroascorbate redox balance increased in a concentration and time dependent manner. This could represent either a slow transient response or a stable increase with regard to plant acclimation to uranium stress. [Copyright &y& Elsevier]